ShaneLogic/driftfusion-device-structure

Driftfusion Device Structure Configuration

Define layer types, configure meshes, and build device structures with graded interfaces.

When to Use

• Specifying device geometry and layer composition
• Setting up spatial discretization for the solver
• Configuring interface properties between material layers
• Defining time mesh for simulation protocols

Layer Type Definition

Define layer_type property for each layer. Four types supported:

1. 'electrode': Pseudo-layer defining system boundaries (not discrete, not visualized)
2. 'layer': Slab of semiconductor with spatially constant properties
3. 'active': Layer flagged as device active layer (stores index in active_layer property)
4. 'interface': Interfacial region between different material layers

Critical Rule: Interface layers MUST be included between materials with different energy levels and eDOS values (at heterojunctions).

Spatial Mesh Configuration

Call meshgen_x to generate spatial mesh:

1. Grid Structure:

   • N intervals (integer points xi)
   • N-1 subintervals (x_sub) where $x_{i+1/2} = (x_{i+1} + x_i) / 2$
   • Solver calculates variables on subintervals, fluxes on integer intervals

2. Mesh Types (xmesh_type):

   • 'linear': Linear piece-wise spacing
   • 'erf-linear': Error function (bulk) + linear (interface) spacing

3. Parameters:

   • layer_points: Points per layer (constraint: ≥ 3)
   • xmesh_coeff: Controls boundary point density (constraint: > 0)

4. Recommendations:

   • Use 'erf-linear' for high ionic defect densities
   • Reduce xmesh_coeff if ionic depletion extends into bulk

Time Mesh Configuration

1. Solver Behavior: pdepe uses adaptive time stepping, interpolates to user-defined mesh

2. Convergence Factors:

   • Weakly dependent on mesh spacing
   • Strongly dependent on tmax and MaxStepFactor

3. Mesh Types (tmesh_type): Linear, logarithmic, etc.

4. Usage Guidelines:

   • Change frequently for intermediate solutions with different timescales
   • Use linear for J-V scans (matches voltage change)
   • Use logarithmic for large time derivatives

Device Structure Building

1. Structure Creation:

   • build_device and build_property create device structures
   • dev: Integer intervals ($x_i$), used for initial conditions
   • dev_sub: Subintervals ($x_{i+1/2}$), used by solver

2. Interface Grading Options (set in build_device):

   • 'zeroed': Property set to zero throughout interface
   • 'constant': Property set to user-defined constant (requires CSV value)
   • 'lin_graded': Linear grading using adjoining layer values (no interface value needed)
   • 'exp_graded': Exponential grading using adjoining layer values

3. Input Requirements:

   • 'lin_graded': Interface value NOT needed
   • 'constant': Interface value IS needed (must be in CSV)
   • Recommendation: Specify all values for all layers to future-proof

Output

• Spatial grids x and x_sub
• Time mesh defined for solver
• Populated dev and dev_sub structures with graded interface properties